1. Field of the Invention
This invention relates to ion thruster engines in general, and in particular, to a circuit that couples to the accelerator grid of an ion engine in such a way that the voltage difference, and hence, the energy contained in a plasma arc occurring between the screen grid and the accelerator grid of the engine, is minimized, thereby reducing or eliminating the damage to the grids caused by such arcing.
2. Related Art
In accordance with well-known Newtonian principles, if electrically charged particles, or ions, are accelerated to a high velocity in a vehicle and then discharged from it, the vehicle will be propelled in a direction opposite to that of the discharged particles, thus giving rise to the development of “electrostatic,” or “ion thruster” engines for space vehicles. While such engines can produce only a very small amount of thrust compared to that produced by the larger and more familiar chemical rocket engines, they are capable of operating continuously for substantially longer periods of time than the latter, and additionally, have a favorably high “specific impulse” (“ISP”) figure when compared to the latter, where ISP is the ratio of thrust to the rate of use of propellant. Thus, while ion engines may lack the large thrust necessary to lift a heavy payload into orbit, they nevertheless have wide application in deep space missions, such as interplanetary exploration, and in orbital satellites for orbital positioning and attitude control, where engine “burns” may last for days, weeks, or even years. Exemplary ion thruster engines are discussed in, e.g., U.S. Pat. No. 4,838,021 to J. Beattie; U.S. Pat. No. 3,156,090 to H. Kaufmann; and, U.S. Pat. No. 3,052,088 to J. Davis et al.
A typical ion engine comprises an ionization chamber having a pair of separate, spaced-apart, fenestrated electrodes, or “grids” having respective, aligned apertures therein, viz., a “screen” grid and an “accelerator” grid, disposed at one end thereof. The two grids are charged with voltages of opposite polarities such that a relatively large voltage potential, and hence, a strong electric field, exists between the two grids. A “propellant,” e.g., gaseous xenon or mercury atoms, is introduced into the chamber, where it is ionized to produce a plasma. The ionized particles form a neutral plasma, which essentially fills the chamber. The propellant ions that pass through the apertures of the screen grid, and thence, into the strong electric field between the two grids, are forcefully accelerated from the chamber through the apertures in the accelerator grid, resulting in a reactive thrust being applied to the ionization chamber.
During operation of the engine, the accelerator grid is normally subject to some erosion caused by “charge-exchange” ions. This erosion is exhibited as a pattern of pits and grooves that occurs on the surfaces of the grids. In addition to this “normal” type of wear of the engine, electrical plasma arcs occasionally occur between the screen and accelerator grids. These arcs are caused by various operational anomalies occurring in the engine, and can cause additional damage to the screen and accelerator grids over and above that caused by the normal charge-exchange ion erosion described above. This additional type of damage to the grids has been shown to make the occurrence of plasma arcs between the grids more frequent by degrading the high voltage integrity of the screen-grid-to-accelerator-grid interface. Such damage can result in a substantial reduction in the reliability and operational life of the engine.
A long-felt but as yet unsatisfied need therefore exists in this field for a reliable, low-cost and lightweight mechanism for reducing the energy contained in a plasma arc occurring between the accelerator grid and the screen grid of an ion engine, thereby minimizing or eliminating the damage caused to the grids of the engine by arcing.